(1) Field of the Invention
This invention relates to an image forming device for transferring a toner carried by a toner carrier like a photoconductive drum onto a paper supported and transported by a transfer belt or directly onto the transfer belt.
(2) Description of the Related Art
There has been a conventional image forming device for pressure-contacting an endless belt on an image carrier for the purpose of image transferring. For example, in order to obtain a two-color image, two photoconductive drums are provided and the transfer belt is pressure-contacted on these photoconductive drums, whereby a toner image formed on the first drum in a first color is transferred onto a paper which has been transported on the transfer belt and then another toner image formed on the second drum in a second color is transferred onto the above paper.
Such device has a construction as shown in FIG. 1 wherein a transfer belt 102 travels around while being pressure-contacted on photoconductive drums 100 and 101 (only 100 is shown in the figure) by two pressure-contacting rollers 103 and 104 (only 103 is shown). The pressure-contacting roller 103 is rotatably supported at both ends thereof by a pair of bearings 103b (only one is shown in the figure). Each bearing 103b is independently energized upward by a spring 105. Since the pressure-contacting roller 104 has the same construction, four springs are provided in all. The pressure-contacting forces between the photoconductive drums 100 and 101 and the rollers 103 and 104 are respectively adjusted by changing the forces of these four springs.
Adjusting a pressure-contacting force so that the transfer belt 102 may be pressure-contacted on the photoconductive drums 100 and 101 is quite important to improve transferring efficiency and obtain a clear image.
Japanese Patent Publication Kokai No. 60-2542167 discloses another example of the conventional image forming devices for getting a two-color image. This is characterized in that the images on two photoconductive drums are transferred directly on the transfer belt.
However, the above-mentioned method of adjusting the pressure-contacting force has the following problems 1), 2), 3), and 4) which restricts the possibility of improving the transferring efficiency and clearness of the images.
1) The pressure-contacting force between the transfer belt and the photoconductive drum and the tension of the transfer belt are usually adjusted after the transfer belt is separated from the photoconductive drum. Accordingly, the adjusted force of the spring and position of the pressure-contacting roller are different from those when the device is in use, namely when the transfer belt is pressure-contacted on the photoconductive drum.
More practically, when the device is in use, the spring is contracted to increase its energizing force and the pressure-contacting roller is moved in a direction to press down the transfer belt to decrease the tension of the belt.
Furthermore, the differences in the energizing force and in the position of the roller between when the device is in use and when the device is in adjustment is varied depending on each device because the position of the photoconductive drum or the like depends on each device.
In consequence, it is extremely difficult to adjust the pressure-contacting force of the transfer belt on the photoconductive drum and the tension of the belt so that they may be most appropriate when the belt is pressure-contacted on the drum, using the above adjusting method.
If the pressure-contacting force of the belt on the drum is not appropriate, the transferring efficiency is lowered whether the image is transferred onto the paper or directly onto the transfer belt. Especially when a plurality of photoconductive drums are provided to obtain a multiple-color image, the transferring efficiency varying by color results in a drastic decline of the image quality.
If the tension of the transfer belt is smaller than the desirable level, the belt slips on the driving roller for driving the belt, whereby the belt cannot travel around smoothly, resulting in malfunctions like transfer error. If the tension of the belt is larger than the desirable level, on the other hand, the belt is given excessive load and its life is shortened.
2) A solution of the above problem is adjusting the pressure-contacting force and the tension of the belt without separating the belt from the drum. This method is not practical since it makes the adjustment itself quite difficult and troublesome and requires high technique.
3) Since the pressure-contacting roller is moved in a direction to press up the transfer belt during adjustment, the transfer belt is given higher tension than in use, whereby being worn rapidly.
4) Since the adjustment is done at four points, it is time-consuming and requires a lot of steps.